1. Field of the Invention
The present invention relates to a current-output type digital-to-analog converter which generates and outputs an output electric current according to an input digital signal.
2. Description of the Related Art
Recently, in a field of compact information terminals such as a portable telephone and PDA (Personal Digital Assistance), there is a device requiring higher voltage than an output voltage of a battery, such as a light emitting diode (hereinafter referred to as LED) used in a back light of a liquid crystal display. In such compact information terminals, a lithium ion battery is frequently used. The output voltage of lithium ion battery is usually about 3.5V, and about 4.2V even in a fully-charged state. However, the voltage higher than the battery voltage is required as the drive voltage of an LED. In the case where the voltage higher than the battery voltage is required, the battery voltage is boosted to obtain the voltage necessary to drive a load circuit such as an LED using a booster-type power supply device in which a switching regulator or a charge pump circuit is used.
When an LED is driven by the booster-type power supply device, a constant current circuit is connected onto an LED driving path to keep a current flowed through LED constant, thereby stabilizing control of emission brightness. To achieve this, a method, of monitoring a voltage at a connection point between an LED and generating a drive voltage of the LED to keep the voltage being monitored constant, may be applied (see Japanese Patent Application Laid-Open No. 2004-22929).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2004-22929    [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-330072
Here, the way to switch the current generated by the constant current circuit according to an input digital value in order to adjust the brightness of LED is considered. Generally, digital-to-analog conversion is performed on the digital value to convert the digital value into the voltage, and the voltage-to-current conversion is performed on the voltage to generate the desired constant current. That is, the constant current circuit functions as a current-output type digital-to-analog converter.
However, in the constant current circuit, when the voltage-to-current conversion is performed, the input voltage of the voltage-to-current conversion circuit varies in a wide range according to the digital signal. Therefore, there is a problem that good linearity is required in the wide range of input voltage for in voltage-to-current conversion circuit. In many cases, the voltage-to-current conversion circuit is designed using an operational amplifier, and it is very difficult to keep good linearity of the operational amplifier in the wide range of input voltage. Accordingly, in the current-output type digital-to-analog converter of the above-mentioned type, there is a problem that conversion accuracy is decreased.